Imaging the position-dependent 3D force on microbeads subjected to acoustic radiation forces and streaming

Abstract

Acoustic particle manipulation in microfluidic channels is becoming a powerful tool in microfluidics to control micrometer sized objects in medical, chemical and biological applications. By creating a standing acoustic wave in the channel, the resulting pressure field can be employed to trap or sort particles. To design efficient and reproducible devices, it is important to characterize the pressure field throughout the volume of the microfluidic device. Here, we used an optically trapped particle as probe to measure the forces in all three dimensions. By moving the probe through the volume of the channel, we imaged spatial variations in the pressure field. In the direction of the standing wave this revealed a periodic energy landscape for 2 μm beads, resulting in an effective stiffness of 2.6 nN m(-1) for the acoustic trap. We found that multiple fabricated devices showed consistent pressure fields. Surprisingly, forces perpendicular to the direction of the standing wave reached values of up to 20% of the main-axis-values. To separate the direct acoustic force from secondary effects, we performed experiments with different bead sizes, which attributed some of the perpendicular forces to acoustic streaming. This method to image acoustically generated forces in 3D can be used to either minimize perpendicular forces or to employ them for specific applications in novel acoustofluidic designs.

Acoustically Driven Fluid and Particle Motion in Confined and Leaky Systems

“…The classical treatment of the associated phenomena has largely been limited to simple, idealized geometries and in comparison, little attention has been given to the combination of acoustics and fluidics in closed geometries. While there have been many recent reports on the physics in confined resonant chambers [2][3][4][5], the physical understanding of acoustically-driven fluid and particle motion in confined leaky systems is limited. These systems are characterized by an acoustic impedance mismatch, between wall material and fluid, that allows a large fraction of the acoustic waves in the fluid to be transmitted to the walls, thereby precluding the build up of acoustic resonances.…”

Acoustic Tweezing and Patterning of Concentration Fields in Microfluidics

“…By actuating the transducer at a resonance frequency of the cavity, an acoustic standing-wave field can be established in the channel cross section in the y-z plane, which is typically a few hundred micrometers in the width W and height H, leading to fundamental resonance frequencies on the order of 1-10 MHz. These systems are well characterized [4,[25][26][27][28] and are used in various biomedical applications, for example, the enrichment of circulating tumor cells in blood [14,29].…”

Two-Dimensional Mapping Separating the Acoustic Radiation Force and Streaming in Microfluidics

“…3(b) shows that the ARF pushed particles toward the nodal lines located every half wavelength, 1 2 λ = 150 μm. Acoustophoretic motions of the 5.0-μm particles are almost entirely ARF governed [20,25,26]. The FOA a r (x, y) determined by MRμPIV is independent of the particle size and should be equivalent to a r (x,…”